The use of industrial robots for the flexible automation of industrial processes has become increasingly common for replacing time consuming, monotonous and difficult work. Such work can, for example, be transferring pieces of chocolate or similar objects from a conveyor belt to places in predetermined locations in, for example, boxes, with high speed and precision where the object is moving on a separate conveyor belt. The ability to be able to handle small and delicate objects effectively with great speed and precision is much sought after in the automation of industrial processes. For this purpose, and for others, an industrial robot has been designed according to the so-called delta concept, where the objective is to achieve rotation of lifted objects to a parallel robot structure. This type of robot, a so-called delta robot, can, for example, be placed in a frame construction above a conveyor belt, and have an arm system to which tools intended to rotate in space, i.e., rotate with three degrees of freedom in x, y and z directions, can be attached. By rotating the robot arm as mentioned later below, it is intended that the robot arm also performs a rotational movement. The arm system commonly comprises a base section, a movable plate and, between them, several jointed pull rods. It is primarily the pull rods that take up the load. A telescopic axle, also called a fourth axle, is arranged between the base section and the moveable plate. Its task is to act as a driving axle from a motor in the robot structure to a tool arranged on the moveable plate. The rotation should have minimal loose play and be able to be carried out in free space, i.e., the distance from the moveable plate to the base of the robot, the base section, is variable. The linear movement can have a speed up to 10 m/s. The number of “pick-ups” of objects can be 120 pieces per minute, which is equivalent to 2 per second. In addition, this delta robot is usually equipped with a system that visually identifies defective objects and only selects those that are perfect.
One problem with this robot is that friction and loose play arises between the integral parts during the rotational movement of the arm system. The telescopic axle must respectively decrease or increase its length at the same time as it is to transfer relatively large moments with great precision, high speed/acceleration, irrespective of whether the object to be picked up is placed at random on the conveyor belt or whether its position is controlled. With known telescopic arms for industrial robots according to the delta concept, an outer tube is joined to an inner axle with an ordinary sliding joint, for example, in the form of splines or similar means, which leads to loose play in the transfer of moments and that is not able to handle moments greater than about 0.5 Nm at moderate speeds.
An industrial robot according to the delta concept with an arm system that comprises a telescopic axle is previously known from U.S. Pat. No. 4,976,582.